Method and apparatus for electrochemically decomposing inorganic materials contained in an aqueous radioactive waste solution

ABSTRACT

The invention is directed to a method for electrochemically decomposing  iganic material contained in an aqueous radioactive waste solution. The solution is concentrated by evaporation in order to reduce its volume for further treatment as a waste solution. In order to achieve a more effective decomposition of inorganic compounds in the aqueous radioactive solution, an electrolytic process is performed during evaporation of the waste solution with the electrolytic process effecting the electrochemical decompostion. An apparatus for carrying out the method includes an evaporator for accommodating the waste solution and the evaporator is configured as an electrolytic cell.

This is a division of application Ser. No. 264,615, filed Oct. 31, 1988,now Pat. No. 4,849,074.

FIELD OF THE INVENTION

The invention relates to a method for electrochemically decomposinginorganic materials contained in an aqueous radioactive solution which,for further treatment as a waste solution, is concentrated byevaporation to reduce its volume.

BACKGROUND OF THE INVENTION

Radioactive liquids such as decontamination liquids or other aqueousradioactive waste solutions are often subjected to electrochemicaldecomposition as described in German Patent No. 2,449,588. Thesubsequent evaporation of the liquids which is intended to produce asubstantial reduction in the quantities of waste which finally have tobe stored can then be carried out with the minimum of difficulty. Inthis connection, reference can be made to European patent publicationNo. 01 62 356 Al.

Electrochemical decomposition of organic and inorganic materialscontained in radioactive aqueous solutions has proved its value withrespect to many such solutions.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a method of the kinddescribed above by means of which a more effective decomposition ofinorganic compounds of aqueous radioactive solutions is achieved. It isa further object of the invention to provide such a method which is lesscomplex.

The method of the invention is for electrochemically decomposingcompounds of inorganic materials contained in an aqueous radioactivewaste solution. The method includes the steps of: passing theradioactive solution into a tank-like evaporator configured as anelectrolytic cell to provide a liquid volume of the solution in theevaporator; and, electrochemically decomposing the compounds in the cellwhile simultaneously vaporizing the liquid volume to provide adistillate and a concentrate for further treatment.

Thus, according to a feature of the method of the invention, anelectrolytic operation is conducted to effect the electrochemicaldecomposition while the waste solution is evaporated.

Electrochemical treatment of the liquid phase in the evaporator canresult in an effective decomposition of disturbing compounds in theparticular radioactive solution in question. For example, inorganiccompounds of low vapor pressure can be electrochemically decomposed toproduce inert gas and H₂ O. Simultaneous electrochemical treatment andevaporation of the radioactive solution simplifies carrying out theprocess.

The invention also relates to an apparatus for carrying out the methoddescribed above. According to a feature of the apparatus of theinvention, an evaporator which receives the waste solution is configuredas an electrolytic cell. The integrated configuration of an evaporatoras an electrolytic apparatus facilitates the process cycle and reducesthe cost of the equipment.

In an advantageous further embodiment of the invention, the evaporatorhousing is connected as a cathode and anodes are provided in theevaporator which project into the volume of liquid.

In a further advantageous development of the invention, a feed pipe forthe waste solution, which is to be evaporated, projects into theevaporator housing and is arranged so that its outlet end is just abovethe housing bottom. The fresh waste solution is supplied far beneath thesurface of the liquid which is exposed to evaporation. The freshsolution can then rise at the anodes and so be subjected to anelectrochemical decomposition. Thus, the waste solution has adequatetime for electrochemical decomposition while electrochemicallydecomposed solution which has already risen can evaporate at the sametime.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described with reference to the single figureof the drawing which is a schematic of the apparatus according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The evaporator comprises a cylindrical evaporator housing 1 of highgrade steel which is connected as a cathode. A concentric anode 3 isdisposed in the liquid chamber of the evaporator 1. Preferably, theanode material consists of titanium and it may be constructed as anexpanded or perforated plate.

Disposed in the upper region of the evaporator housing 1 is a feed pipe4 for the solution which is to be evaporated. The pipe 4 projects intothe solution and is disposed with its outlet end 2 just above thebottom. Laterally on the evaporator housing 1, a pipeline 5 extends inthe lower region horizontally to a heat exchanger 7 and extends fromthere back into the evaporator so as to be below the surface 9 of theliquid in the evaporator. The heat exchanger 7 is heated by a heatingmedium which is supplied via coiled pipes 11. Natural circulation isprovided within the pipeline 5 connected to the heat exchanger 7.

An ancillary electrode in the form of an anode core 13 is disposedinside of the lower portion of the pipeline 5.

The evaporator 1 extends upwardly into a connecting piece 15 which isprovided with a demister 17 for trapping aerosols. The vapor is passedinto a cooler 21 via a pipeline 19. The distillate is drawn off via apipeline 23.

The apparatus described above functions in the manner explained below.

A radioactive solution containing inorganic materials is fed into theevaporator housing 1 via the feed pipe 4. An electrical voltage isapplied across cathode 1 and anode 3 via connections 25 and 27. Anelectrical voltage is also applied across cathode 1 and core anode 5which increases the electrochemical decomposition activity. The controlof the electrolytic process is performed by maintaining the current orvoltage constant. The radioactive solution is heated by the heatexchanger 7 to a temperature of about 100° C. The solution vaporizesonce the inorganic materials contained therein have beenelectrochemically decomposed. The distillate is drawn off from thecooler 21 and passed to the further units in the process. A concentratedsolution is left in the evaporator which is likewise discharged forfurther treatment.

The radioactive solution fed into the evaporator housing can be, forexample, a nitric acid solution.

It is understood that the foregoing description is that of the preferredembodiments of the invention and that various changes and modificationsmay be made thereto without departing from the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:
 1. An apparatus for electrochemically decomposingcompounds of inorganic materials contained in an aqueous radioactivewaste solution, the apparatus comprising:an evaporator for receiving andvaporizing said aqueous radioactive waste solution to form a distillateand concentrate for further treatment; and, said evaporator includingelectrolytic means for electrochemically decomposing said compoundswhile said waste solution is in said evaporator.
 2. The apparatus ofclaim 1, said evaporator comprising a housing defining a tank forholding a volume of said solution; and, said electrolytic meanscomprising a cathode connection applied to said housing and an anodeprojecting into said solution.
 3. The apparatus of claim 2, comprising afeed pipe having an outlet for passing the radioactive waste solutioninto said tank; and, said feed pipe being mounted in said housing toplace said outlet close to the bottom of said tank so as to permit theincoming solution to pass along said anode to effect the electrochemicaldecomposition of said compounds while rising to the surface of saidvolume.
 4. An apparatus for electrochemically decomposing compounds ofinorganic materials contained in an aqueous radioactive waste solution,the apparatus comprising:a tank for receiving a liquid volume of theaqueous radioactive waste solution; feed means for feeding additionalquantities of said solution into said tank beneath the surface of saidliquid volume; electrolytic means for electrochemically decomposing saidcompounds; said electrolytic means including means for connecting saidtank as a first electrode of a first polarity and a second electrode ofa second polarity opposite to said first polarity; said second electrodebeing disposed in said liquid volume above said feed means; and, heatingmeans for heating said liquid volume so as to cause said solution tovaporize at said surface.
 5. The apparatus of claim 4, said heatingmeans including: a conduit having a first end penetrating said tank andcommunicating with said liquid volume at a first elevation above thebottom of said tank and having a second end penetrating said tank andcommunicating with said liquid volume at a second elevation above thebottom of said tank with said second elevation being less than saidfirst elevation; and, heat exchanger means disposed in said conduit forheating the liquid in said conduit to thereby provide a natural heatinduced circulation between said conduit and said tank thereby raisingthe temperature of said liquid volume to effect the vaporization thereofat said surface.
 6. The apparatus of claim 5, said electrolytic meansfurther comprising an ancillary electrode of said second polaritydisposed in said conduit.